1. Field of the Invention
This invention relates to the use of electronic trip circuit breakers, particularly the use of a zone selective interlock system with electronic trip circuit breakers.
2. Description of the Related Art
The primary concern in determining the appropriate circuit protective devices for use in an electrical power distribution system is protection of the distribution system in the event of a fault precipitating abnormal overcurrent condition. The concern is to prevent or at least minimize damage to the system, including its conductors and connected loads. Equipment failure, human error, or emergencies of natural origin may cause such a fault. Typically, such faults are unpredictable, and thus the selected circuit protective devices must function automatically to fully protect the system, and incidentally to protect personnel and property against the consequential hazards of such faults.
Another principle concern with the circuit protection devices to be used is minimizing the extent and duration of electrical service interruption in the event of a fault. Typically, two or more circuit protection devices are placed between a fault and the source of the fault current. In order to minimize electrical service interruption, the protective devices are selective in response such that the one nearest the fault will first attempt to interrupt the fault current. If this protective device does not clear the fault in a timely fashion, the next upstream protective device will attempt to do so, and so on. This response selectivity is termed system selective coordination.
In a circuit breaker having an electronic trip unit, the electronic trip unit has two modes of operation, restrained and unrestrained. In the unrestrained mode, the electronic trip unit initiates a trip of the circuit breaker instantaneously regardless of selected delay settings. In the restrained mode, the electronic trip unit initiates a trip of the circuit breaker after the selected delay has timed out. Incorporating a zone selective interlock system allows the circuit breakers to communicate with each other ensuring that the breaker closest to the fault clears the fault.
Zone selective interlocking functions in an electrical system typically provide for a lower level "downstream" zone to send a restraint signal up to higher level "upstream" zone circuit breakers, wherein the upstream circuit breakers would be restrained from tripping, allowing the downstream circuit breaker to trip to minimize interruption of the electrical system. However, a lot of power is required for the downstream circuit breaker to send the restraint signal up through one or more levels of upstream circuit breakers.
FIG. 1 (labeled prior art) illustrates a typical zone selective interlocking system 100 for electronic trip circuit breakers, including a main (or upstream) circuit breaker A and branch (or downstream) circuit breakers B and C, respectively. The arrows indicate the direction of current flow. The system illustrated in FIG. 1 has separate short time (ST) and ground fault (GF) restraint circuits, however, only the ST connections are illustrated in FIG. 1. Each circuit breaker includes an output terminal (ST OUT), a common output terminal (ST OUT COMMON), an input terminal (ST IN), and a common input terminal (ST IN COMMON). The output circuit must be connected to the input circuit for each circuit breaker in order to provide a self-restraint signal to the circuit breaker. For example, in circuit breaker B, ST OUT is connected to ST IN and ST OUT COMMON is connected to ST IN COMMON to provide a self-restraint signal to the circuit breaker B. Therefore, if a fault occurs downstream from circuit breaker B, circuit breaker B would drive a restraint signal up to circuit breaker A while also driving its own restraint input circuitry and also the restraint input circuitry of circuit breaker C. Therefore, in the system design of FIG. 1, each circuit breaker must restrain its own input as well as the input circuits of all of the circuit breakers at the same level or zone and the input circuits of all the circuit breakers it must restrain at the other levels or zones.
In large multiple level systems, a single output circuit may not have sufficient drive capability to drive a restraint signal to all of the circuit breakers as described. An interface module is typically added to the system to boost drive capability, which adds cost and complexity to the system. The system as illustrated in FIG. 1 also does not include an isolation means and the common inputs are all tied to system ground. Therefore, a ground fault occurring in the system may cause ground currents to flow throughout the zone selective interlocking system possibly damaging circuitry.
Another problem with existing zone selective interlocking systems is ensuring that the system is properly wired. The circuit breakers in the system are typically located great distances apart. The only method for checking existing systems is to monitor the system signals with an oscilloscope while a fault signal is secondarily injected using a test kit or primarily injected using a current supply capable of several thousand amperes. This method causes a number of difficulties such as requiring personnel at multiple locations, a fault must be simulated to cause the restraint circuits to operate, and power may not be available or equipment mobile enough to check different levels of the system.
Accordingly, a zone selective interlocking system for electronic trip circuit breakers is needed which does not use large drive capabilities, which has sufficient isolation between restrained and unrestrained circuit breakers to prevent damage to the circuitry, and which can be easily tested.